1. Field of the Invention
The present invention relates generally to spacecraft and, more particularly, to spacecraft attitude control.
2. Description of the Related Art
A spacecraft typically has an attitude-control system for active control of the spacecraft""s attitude and the attitude-control system often receives attitude-measurement signals from one or more star trackers. Star trackers generally process received star light through focusing optics to a solid state structure such as the structure 20 of FIG. 1A which includes an array 22 that defines a large number (e.g.,  greater than 200,000) of pixels (23 in FIG. 1A) that each receive light into a respective photodiode which converts the incident light into a corresponding electrical charge.
The generated charge at each pixel is then transferred to an edge of the array 22 in a conveyor-belt-like fashion in which the charge moves between each pair of pixels via a charge-coupled device (CCD) that includes a transfer gate of an associated complementary-metal-oxide semiconductor (CMOS) device. Therefore, the array 20 is typically referred to as a CCD array. In addition, charges associated with the CCD array are collectively referred to as a frame and a readout of the frame is referred to as a frame readout or frame transfer.
An exemplary readout structure of generated charges has been shown and described (e.g., see Holst, Gerald C., CCD Arrays, Cameras and Displays, JCD Publishing, Winter Park, Fla., 1998, pp. 59-61) to have horizontally-arranged serial CCD registers 24 positioned at the upper and lower perimeters of the array 20. In this arrangement, the array is divided into four quadrants and each pixel charge of a quadrant is vertically transferred down a CCD column to a respective register 24.
Each time the respective register 24 is filled with the vertically-transferred charges, these charges are then horizontally transferred to a respective sense amplifier 26 which effectively forms an array output port 28. The sense amplifier typically includes a capacitor (e.g., formed by a floating diode) which converts each arriving electrical charge to a corresponding voltage which is then delivered to the output port 28 by the sense amplifier 26.
Although the readout structure of FIG. 1 realizes a rapid frame readout because it assigns a horizontal register 24 and corresponding sense amplifier 26 to each array quadrant, this structure adds to the array complexity. The above-cited reference, for example, also shows simpler readout structures such as one in which an upper half of the array 20 is served by a single upper register that has an output port at the array""s upper right corner and a lower half is served by a single lower register that has an output port at the array""s lower left corner.
FIG. 1B shows pixels 22 of the array 20 that are within an exemplary circle 1B of FIG. 1A. The light from a single star is typically focused on the array as a star image which generates electrical charges in a contiguous group of pixels 23 such as those within the star image 30 of FIG. 1B. The electrical charges will typically be quite small for pixels adjacent the perimeter of the star image 30 and progressively increase with distance from that perimeter. Star trackers generally include a processor that calculates a centroid 32 of the electrical charges within the star image 30 and it is the measured coordinates Cms of this star image centroid that are delivered through the array output ports (28 in FIG. 1A).
The star tracker readout process is susceptible to a number of errors. A first class of these errors concerns the addition of random noise (e.g., background and electronic) which induces temporal noise TN in the readout signals. Centroid jitter is generated as the star image moves across pixel boundaries and this jitter is a primary source of a second class of errors which are typically referred to as high spatial frequency errors Ehsf. Finally, low spatial frequency errors Elsf include calibration residuals, color shift errors and charge transfer efficiency (CTE) errors. Calibration residuals are caused by temperature-induced focal length shifts and color shift errors are caused by chromatic aberration in the focusing optics.
CTE errors are generated because the charge transfers of the array""s CCD devices are not perfect but rather, are determined by the star tracker""s CTE which is defined as the proportion xcex5 of charges that are actually transferred from a trailing pixel to a leading pixel. Accordingly, a fraction 1-xcex5 of charge is left behind in the trailing pixel. As explained in the above-cited reference, charge is not lost but charge is rearranged so that trailing pixels gain charge from leading pixels.
A transfer arrow 34 in FIG. 1C indicates that the charges within the initial star image 30 are in the process of being transferred downward. Because of the CTE-induced effect described above, the trailing edge 36 of the star image 30 has been extended and, as a result, the initial star image centroid 32 has been translated away from the leading edge 38 of the star image 30 to a subsequent position 32S. The difference between the initial and subsequent centroids 32 and 32S is a CTE error which is included in the measured star coordinate Cms that corresponds to the subsequent star image centroid 32S.
Typically, this CTE error is quite small. Scientific-grade star trackers, for example, have CTEs that exceed 0.999999 so that net transfers across thousands of CCDs (to the output ports 28 of FIG. 1A) are quite high (e.g.,  greater than 0.99). In long term spacecraft missions, however, time and incident radiation generally degrade the CTE and this degradation may be sufficient to generate significant errors in a spacecraft""s attitude control system and threaten the survival of the mission.
The present invention recognizes that measured star coordinates Cms of star image centroids include CTE errors which are functions of the CCD path lengths over which the associated electrical charges traveled. In particular, the errors are substantially a product of the path length and a star-coordinate error factor "xgr" which, in turn, is a function of the star image magnitudes msi. The invention further recognizes that information contained in different measured star coordinates Cms can be organized to facilitate the estimation of the star-coordinate error factor "xgr" with conventional estimation processes.
From this recognition, the invention provides structures and methods for deriving corrected star coordinates Ccrctd from measured star coordinates Cms that include CTE errors and, accordingly, provides improved spacecraft attitude control systems.
In a method embodiment, CCD electrical charges are transferred over corresponding first and second paths of a CCD array at respective first and second times that differ by a measurement time interval xcex4t to thereby provide respective first and second measured star coordinates. The measured star coordinates are differenced to form measured star-coordinate differences xcex4Cms and, in addition, a star-coordinate movement xcex4C due to rotation of the spacecraft over the measurement time interval xcex4t is determined.
With the measured star-coordinate differences xcex4Cms, the star-coordinate movement xcex4C and knowledge of a maximum path length Lmax in the CCD array, a composite coordinate-measurement signal Smscomp is formed which substantially equals the sum of the error factor "xgr" and a measurement variance "sgr"ms. The composite coordinate-measurement signal Smscomp is processed in accordance with the star image magnitudes msi to derive an error factor estimate "xgr"* of the error factor "xgr". Finally, the measured star coordinates Cms are corrected with the error factor estimate "xgr"* to thereby realize the corrected star coordinates Ccrctd.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.